This invention relates to a scanning device for a magnetic tape equipment of the kind used for recording and/or reproducing broad band signals.
A scanning device of the above kind is disclosed in German Disclosure Document DE-OS No. 23 40 374 corresponds to U.S. Pat. No. 3,862,355. Such scanning devices are subject to a series of requirements which are in part incompatible, but all of which should be satisfied as far as possible. Thus it is to be regarded as one of the principal requirements of equipment in the magnetic storage art that the relevant data should be recorded or stored at a density as high as possible. In the case where it is required to record frequency modulated television signals having frequencies up to 13 MHz or more, or digital television signals having a bit rate of 50 Mbit/s or more, the amount of data stored per unit surface area of the magnetic tape should be as small as possible. Accordingly attempts have been made to reduce the width of the tracks inscribed upon the magnetic tape and to reduce the spacing distance between the adjacent tracks. However, it is clear that a storage system having greatly reduced track widths and track spacings will be more prone to faulty scanning during reproduction. Therefore all high quality magnetic tape equipment of the tape now being discussed includes electronic circuits for regulating the driving motor for the magnetic tape and/or for the magnetic head-carrying scanning disc to provide the best scanning conditions in reproduction.
The motor control or regulations circuits receive signals representing the instantaneous frequency and phase position of the tape drive roller or the rotating scanning disc from optically or magnetically coded discs or drums which, in all the known magnetic tape equipments, are arranged upon a common shaft with the tape drive roller or the scanning disc and therefore rotate with them in synchronism. Since, as already indicated, all magnetic tape storage systems using high density storage are particularly sensitive in respect of deviation of the magnetic scanning heads from the recorded track (tracking errors) it is necessary that these coded discs or drums (usually referred to as tacho discs) should as far as possible be free from periodic errors and should be aligned in correct angular relationship with the scanning disc.
A further requirement for scanning devices for broad band signals is to permit the rapid and economic replacement of the relevant parts of scanning devices which have become unserviceable due to wear or damage suffered by the magnetic heads when in scanning contact with the magnetic tape. On account of the extreme accuracy with which the position of the magnetic heads must be located on the periphery of the scanning disc, and on account of the high degree of concentricity required when centering the scanning disc itself upon its shaft, and finally on account of the correct angular alignment demanded for the tacho disc with respect to the scanning disc, it has in the past always been the case that the responsibility for the repair of unserviceable scanning devices rested with the manufacturer, who had available the necessary adjusting and testing devices. However, this is time consuming and expensive.
In a practical magnetic tape equipment for the recording and reproduction of broad band signals according to the oblique track method, high precision components are provided upon the shaft for accommodating the scanning disc carrying the peripherally mounted magnetic heads, which components provide, during the mounting of the scanning disc, the necessary degree of axial, radial and polar orientation. Consequently in the event of damage or wear being suffered by the magnetic heads the scanning disc itself can be independently replaced. This method of replacement functions satisfactorily in practice. On account of the random pairing of scanning discs and the other devices during assembly of the scanning device it is, however, necessary that all the relevant tolerance ranges must be very closely maintained so that in the event of an exceptional summation of several tolerances in one direction, satisfactory orientation of the scanning disc is still ensured. One example of the concurrence of precise tolerances relates to the circumpolar alignment of the scanning disc with respect to the tacho disc on the same shaft. In this case errors of eccentricity of the two discs can be additive and can result in deviations from synchronous running of the rotating shaft. The very high working accuracy demanded for the machining of the reception surfaces of both of the discs so as to avoid such errors increases the cost of manufacture of the scanning device.